It is known to coat metallic, normally ferrous, filaments such as wire or strip or sheet with molten metals such as zinc, aluminium and zinc/aluminium alloys. The filament is passed through a bath containing the coating metal in a molten condition. After leaving the bath a wiping force is applied to the filament to remove excess coating metal from its surface and to impart a smooth surface to the coating metal remaining on the filament.
It has been known to apply a mechanical wiping force to the filament by a number of methods. In one method wiping pads of asbestos or similar are used to physically wipe excess coating material from the surface. In a second method the filament is passed upwardly through a granular layer of materials such as charcoal, pebbles and glass beads with or without a lubricant such as oil or tallow, which layer floats on the surface of the molten metal bath. Another wiping method is gas jet wiping in which the filament passes through a stream of a suitable gas such as air, nitrogen or steam which applies a wiping force to the filament. Proposals have also been made to apply an electromagnetic wiping force to the filament.
The performance of the granular layer wiping method was improved by the injection of a reactive gas such as hydrogen sulphide into the granular layer in a process known as gas wiping and described more fully in Australian patent specification 421,751. In this process the primary purpose of the reactive gas is to form a layer of metal sulphide on the metal bath and within the granular layer to assist in the physical wiping of the excess metal from the filament.
It has subsequently been proposed to inject a reactive gas into a container, which surrounds the granular layer and at its lower end projects into the metal bath, at a level above the granular layer in order to improve the appearance of the wire (see British patent specification 1,446,861). In a later development it has been proposed to stabilise the surface of a molten metal coating on a filament being wiped by an electromagnetic force by injecting a reactive gas into a container which surrounds the electromagnetic device and which projects into the metal bath (see British patent specification 2,010,917). In each of these cases the reactive gas has been applied within a container positioned directly above, and in contact with the metal bath. This has both prevented the loss of reactive gas from the bottom of the container and has applied the reactive gas to the filament before there has been any possible oxidation of the coating metal.
After the filament has been coated and wiped, it is necessary to solidify the coating metal before it comes into contact with a solid object. Solidification of the coating metal is normally achieved by passing the filament through a cooling fluid, normally water and/or air. It has been found, in the gas jet wiping process, that it can be difficult to cool the filament without causing the resultant coating to have a rough surface. It has also been found that the solidified coating has a dull appearance; both of these characteristics are undesirable.
It has now been found that surprisingly beneficial results can be obtained by the application of a reactive gas to a filament that has been hot dipped and wiped by the gas jet wiping method. The advantages of the present invention are that it is possible to reduce, and in some cases eliminate, surface defects previously observed with gas jet wiped filaments cooled by the direct application of a cooling fluid, and to also give to the filament a relatively shiny lustre. It would not have been expected that the use as a reactive gas atmosphere would have been applicable to gas jet wiping. By its very nature in the gas jet wiping method the jet wiping nozzle is spaced from the metal bath. The container to hold the reactive gas must be positioned above the gas jet wiping nozzle and must have an aperture in its bottom to receive the filament. The process according to this invention thus involves the use of an effectively open bottomed gas container. The container will also be spaced sufficiently above the metal bath that some oxidation of the molten metal coating may occur prior to the wire being contacted by the reactive gas.
The present invention consists in a method for the coating of a metallic filament with a molten metal comprising the steps of drawing the filament from a molten metal bath, passing the filament through a gas jet wiping nozzle having a gas orifice spaced apart from the molten metal bath to direct a wiping gas stream against the filament to wipe excess molten metal from the filament, passing the wiped filament through a gas containment vessel containing a reactive gas atmosphere including sulphide or chloride radicals or materials which will decompose to produce such radicals, the containment vessel being spaced from the gas jet wiping nozzle sufficiently to allow the venting of wiping gas therebetween such that the reactive gas is not adversely diluted and the gas containment vessel being sufficiently long that the filament has a long enough residence time in the container to allow the reactive gas to react with the molten metal on the filament, and then cooling the filament by applying thereto a fluid coolant.
In another aspect the present invention consists in apparatus for the coating of a metallic filament with a molten metal comprising a molten metal bath, means to draw a filament from the molten metal bath, a gas jet wiping nozzle having a gas orifice spaced apart from the molten metal bath and adapted to direct a wiping gas stream against the filament to wipe excess molten metal from the filament, a gas containment vessel containing a reactive gas atmosphere which includes sulphide or chloride radicals or materials which will decompose to form such radicals, the containment vessel being spaced from the gas jet wiping nozzle sufficiently to allow the venting of wiping gas therebetween such that the reactive gas is not adversely diluted and the containment vessel being sufficiently long that a filament passing therethrough will have a residence time in the containment vessel long enough to allow the reactive gas to react with the molten metal on the filament, and cooling means adapted to apply a cooling fluid to a filament after it has emerged from the gas containment vessel.
The present invention enables filaments of acceptable surface quality to be produced over a wider range of conditions than has previously been possible with gas jet wiping. It has been found that, depending on the shape of the filament, the thickness of the coating metal and the flow rate of the cooling fluid, there is a speed of passage of the filament above which the degree of the filament's surface smoothness is unacceptable (which term is taken to mean that the roughness can be felt by scraping ones fingernail longitudinally along the filament) if the invention is not used. The flatter the filament is (i.e. the larger its radius of curvature), and consequently the greater the resistance offered to the flow of the cooling fluid, the slower the filament must be processed in order to achieve acceptable surface quality. Larger thicknesses of coating metal and higher flow rates of cooling fluid, also require slower process speeds in order to produce acceptable levels of surface smoothness. By way of example, it has been found that if a 4.00 mm diameter wire with a molten metal coating thickness of greater than 0.04 mm, is passed through a stream of water jets (each jet with a cross-sectional area of 2 cm.sup.2 and flow rate of 6 liters/minute) the wire will have an unacceptable surface smoothness when processed at speeds above 0.8 m/s. For a 2.50 mm diameter wire under the same conditions the coating quality is unacceptable at speeds above 1.2 m/s. There is also a range of speeds up to this, in which the coating quality progressively deteriorates from being perfectly smooth to unacceptable.
The filament is preferably ferrous wire or rod however the process is also applicable to tubular products, strip products whether planar or shaped in cross-section and to sheet products. The coating metal is preferably zinc however other coating metals such as zinc alloys containing a majority of zinc may also be used.
The jet wiping nozzles for use in the present invention may be any one of the conventional jet wiping nozzles known, for example, from the following patent specifications:
U.S. Pat. No. 2,194,565;
U.S. Pat. No. 3,060,889;
U.S. Pat. No. 3,270,364;
U.S. Pat. No. 3,459,587;
U.S. Pat. No. 3,533,761;
U.S. Pat. No. 3,611,986;
U.S. Pat. No. 3,707,400;
U.S. Pat. No. 3,736,174;
Australian Pat. Nos.
458,892;
537,944;
539,396;
544,277;
It is preferred, however, to use the jet wiping nozzle the subject of the present applicant's copending Australian patent application No. PJ 0032 entitled "Improved Product and Process" the contents whereof are hereby incorporated in this specification by reference. The wiping gas may be an oxidising gas such as air or, preferably, a non-oxidising gas such as nitrogen.
The containment vessel should be spaced apart from the gas jet wiping nozzle sufficiently for that part of the wiping gas stream that flows in a direction away from the metal bath to be adequately vented between the nozzle and the containment vessel to such an extent that the reactive gas is not adversely diluted. If the two are too close together the wiping effect of the gas jet nozzle may be deleteriously affected and wiping gas entering the containment vessel through the aperture admitting wire into the vessel may adversely affect the formation of a stabilising film on the filament through dilution of the reactive gas. On the other hand some outward pressure from the wiping gas jet may prevent an undue flow of the reactive gas atmosphere out through the aperture which admits the filament into the vessel.
The cooling means may be any one of a number of known types wherein a stream of water or other liquid or a stream of a cooling gas is caused to contact the filament and its still molten coating. The preferred cooling means is that described in Australian Patent Specification 462,301 the contents whereof are incorporated herein by reference.
An air knife is preferably positioned between the reactive gas containment vessel and the cooling means to direct a stream of air across the wire. This air knife serves to prevent droplets of water from dropping into the molten metal bath or from running down the strand if for any reason it is necessary to stop the strand temporarily.
The reactive gas preferred is hydrogen sulfide however any gas that contains or provides the sulphide or chloride radical may be used. For example, chlorine, hydrogen chloride, diethyl disulfide, dipropyl disulfide, dimethyl disulfide, ethyl mercaptan, propyl mercaptan, carbon disulfide, methyl mercaptan and any similar gas.
The reactive gas atmosphere is preferably comprised of reactive gas in a combustible carrier gas such as natural gas, liquified petroleum gas, or propane. The use of such a combustible carrier which can be burnt as it passes out from the gas containment vessel is particularly useful when the reactive gas is hydrogen sulphide or a mercaptan as the sulphide containing material can be combusted together with the combustible gas.
The reactive gas is preferably present in the reactive gas atmosphere in concentration by volume of greater than 0.01%, more preferably 0.5% to 1.5%. The reactive gas containment vessel should be of sufficient length to allow reaction to take place between the reactive gas and the molten metal and to form a protective film on the molten wire. It has been found, for instance, that a containment vessel having a length of 15 cm is satisfactory for the galvanising of a 2.5 mm diameter steel wire at a speed of up to 1.5 m/s at a coating 300 g/m.sup.2 and a hydrogen sulphide concentration of 0.5% by volume. If a larger diameter wire is to be treated or a faster speed or larger cooling mass is desired then a longer gas containment vessel is required.